Time-reversal symmetry breaking and spontaneous Hall effect without magnetic dipole order

Spin liquids are magnetically frustrated systems, in which spins are prevented from ordering or freezing, owing to quantum or thermal fluctuations among degenerate states induced by the frustration. Chiral spin liquids are a hypothetical class of spin liquids in which the time-reversal symmetry is macroscopically broken in the absence of an applied magnetic field or any magnetic dipole long-range order. Even though such chiral spin-liquid states were proposed more than two decades ago, an experimental realization and observation of such states has remained a challenge. One of the characteristic order parameters in such systems is a macroscopic average of the scalar spin chirality, a solid angle subtended by three nearby spins. In previous experimental reports, however, the spin chirality was only parasitic to the non-coplanar spin structure associated with a magnetic dipole long-range order or induced by the applied magnetic field, and thus the chiral spin-liquid state has never been found. Here, we report empirical evidence that the time-reversal symmetry can be broken spontaneously on a macroscopic scale in the absence of magnetic dipole long-range order. In particular, we employ the anomalous Hall effect to directly probe the broken time-reversal symmetry for the metallic frustrated magnet Pr"2Ir"2O"7. An onset of the Hall effect is observed at zero field in the absence of uniform magnetization, within the experimental accuracy, suggesting an emergence of a chiral spin liquid. The origin of this spontaneous Hall effect is ascribed to chiral spin textures, which are inferred from the magnetic measurements indicating the spin ice-rule formation.